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Insulating a cathedral ceiling

Jon Cypert | Posted in Energy Efficiency and Durability on

I am trying to find an effective way to insulate a cathedral ceiling (with collar ties) in zone 5 south of Chicago. I have read many of the postings and have gathered much information, but am still hesitant due to the moisture drying to one side issue.

What I have is 2×6 rafters on a 4/12. I am doing a complete remodel and plan on removing the existing drywall and insulation. I want to verify the existence (or not) of ventilation chutes from eave to ridge.

I will be performing all of the work interior so as not to incur the expense of a new roof. I am going to pitch my thoughts and would welcome any feedback.

Once I have opened up the ceiling from the interior, my thoughts were to apply 1-by strips to the rafters against the roof decking to allow for some type of vent chute application up to the eave. Next, I thought I would strip out the underside of the rafter using 2×4 on edge screwed and glued to the existing 2×6. This would give me about 8 inches of useable cavity.

It is at this point I start questioning the many options available to me. Do i forego the vent chutes and simply spray closed foam on the underside of the roof sheathing, if so at what thickness? Do I utilize a firm vent chute made from some type of thin paneling/mdf/plywood and fill the remainder of the cavity with batt insulation (what type?) then fasten 2″ thick extruded poly to the underside of the rafters (drywall to follow)?

It is my understanding that I don’t want to make a sandwich using batt or blown between rigid foam board. Could someone give me some advice on what would be acceptable in regards to moisture control?

I understand I may not be able to acheive the R-value that the building code requires but I would like to acheive the best bang for my buck considering what I have to work with. Any thoughts or suggestions would be greatly appreciated.
Jon Cypert [email protected]

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Replies

  1. John Klingel | | #1

    Have you read this? https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling I hear the guy knows what he is talking about.

  2. GBA Editor
    Martin Holladay | | #2

    Jon,
    After you have read the article that John Klingel linked to, let us know which of your many questions remain unanswered. It's best to ask only one or two questions per post -- that way you are more likely to get answers.

  3. Eric Novotny | | #3

    I am guessing the cathedral ceiling is currently vented (i.e. soffits/ridge) if you are inspecting for a proper vent chute.

    Even with furring out the interior with 2x4's, you are going to have a very hard time getting near code required R-Value with vented assembly (allowing for a 1" vent depth).

    I think you are going to really need spray foam in this application to get near the required R-Value.

  4. Bri Kahler | | #4

    I two have the same problem , have a cathedral ceiling vented with 2by6 rafters. What was your final approach? I was thinking of 1 1/2 furring strips to the inside of the rafters to the deck side to create my vent space. Then use 1 inch xps non faced foam board pushed up into the rafter, this will give me a r-5 to start. Than use roxul unfaced r-15 under the foam. To further seal my ceiling I would finish with a full skin of polyiso foil faced foam to the underside of the rafters and tape all seams to create a continuos seal on the inside ceiling. Then finish my ceiling with t and g pine. Will this create a vapor sandwitch? I will get about r-23 or 25 with this ,, not a year round structure but would like to get as much r as possible

  5. Aj Builder, Upstate NY Zone 6a | | #5

    BRI.... Silly. Inserting rigid foam is a crappy answer to crap to start with IMO.

    Air leaks are your worst heat loss. Read here many threads for a bit.

  6. Aj Builder, Upstate NY Zone 6a | | #6

    Jon, yes you have a good plan.

    Add depth. You could add foam strips to get more depth with less thermal bridging.
    Vent, check.
    Add dense batts, check.
    Add taped rigid, check.
    Done.

    Air seal well.

    Code R is not the goal. More effective R is the goal along with comfort. More is better, but budgets and space matter too.

  7. Bri Kahler | | #7

    Crappy? Well I guess i have to polish a Terd then. Any real answers out there?

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Bri, start your own question thread, some pics, and maybe Martin and Dana will set you up. I need more info that much I know.

    As you can tell fitting foam between framing members is not my thing. I think it is the silliest idea ever thought up for too many reasons among other problems with your idea.

  9. GBA Editor
    Martin Holladay | | #9

    Bri,
    Your plan will work, although the final R-value will be less than minimum code requirements.

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